MECHANICAL SEPARATION 297 pressure which is being maintained at so many pounds above atmospheric. In vacuum work we have the residual pressure due to maintaining the vacuum and on the other side the atmospheric. In other words the compressor is working against a pressure equal to gage pressure converted into pounds. At low pressures there is very little difference in the horsepower required to compress air at sea level or high altitudes. For the majority of vacuum problems 6 to 7 hp. per 100 ft. of vacuum displacement will cover the filter, the vacuum compressor, the pressure compressor and the centrifugal pump or about 5 hp. for each 100 ft. of filtering surface. The pressure compressor requires much less power than the vacuum machine. In many filtration problems it is only required for the occasional blow necessary to free the filter medium of particles which have clogged it and the small use of air below the scraper to the same end, for blowing the cloth after a shutdown or for stirring up the material in the filter tank after a shutdown. Its use to blow the cake before scraping increases the moisture as it tends to blow through a part of the moisture adhering to the burlap and screen under the filtering medium and its use at this point should be avoided. The power to drive the filter is small, less than 1 hp. on machines under 12 by 12 size. Filter drums revolve from J^ to Ko r.p.m. The American filter consists of a series of disc leaves placed close together and mounted on a revolving shaft. The Robacher filter uses "filtros," an artificial porous stone as the filtering medium and mounts the sections on the sides of a rotating drum. Costs.—Ten cents per ton would represent about an average cost for vacuum filtering including preliminary thickening. The cost of disposing of the product made will average about 20 cts. per dry ton. This figure is the cost of disposal by hand labor or loading for shipment. Where the cake is of no value or is subjected to further treatment mechanical disposal will make the cost figure very much lower. Pressure or Intermittent Filtration.—The disadvantage of this mode of filtering lies in the intermittency of operations and the tendency to mussiness about the filter with consequent loss of filtrate and cake. The loss of capacity is offset to some extent by the ability to form a heavier cake. The principal advantage of pressure filters is in washing. Given any fixed and necessary amount of washing the pressure filter system can be arranged to suit. In the vacuum systems washing can only be performed between the time the drum rises out of the tank and until it reaches the scraper. It makes no difference whether the drum is run slowly or fast. If the drum is run slowly the cake is heavier and the rate of percolation through it of the wash is slower. It is useless to turn any more wash liquid on the vacuum cake than will be taken care of by the rate of percolation through. Volatile wash liquids cannot be used on vacuum filters without serious loss by evaporation or loss of one of its principal advantages, the ability of seeing the operation of filtering. On solid materials which hold moisture with great pertinacity or through which liquids percolate very slowly the vacuum filter does not work successfully and pressure filtering is required. Ries1 cites a natural clay from Wyoming which when placed in a measuring flask basorbed and retained sufficient water to increase it bulk eightfold. Some "! Clays, Their Occurrences, Properties and Uses," JOHN WILEY & SONS, New York, 1906, p. 128.